In the field of semiconductor technology, research and development efforts are continued for further miniaturization of pattern features. Recently, as advances including miniaturization of circuit patterns, thinning of interconnect patterns and miniaturization of contact hole patterns for connection between cell-constituting layers are in progress to comply with higher integration density of LSIs, there is an increasing demand for the micropatterning technology. Accordingly, in conjunction with the technology for manufacturing photomasks used in the exposure step of the photolithographic microfabrication process, it is desired to have a technique of forming a more fine and accurate circuit pattern or mask pattern.
In general, reduction projection is employed when patterns are formed on semiconductor substrates by photolithography. Thus the size of pattern features formed on a photomask is about 4 times the size of pattern features formed on a semiconductor substrate. In the current photolithography technology, the size of circuit patterns printed is significantly smaller than the wavelength of light used for exposure. Therefore, if a photomask pattern is formed simply by multiplying the size of circuit pattern 4 times, the desired pattern is not transferred to a resist film on a semiconductor substrate due to optical interference and other effects during exposure.
Sometimes, optical interference and other effects during exposure are mitigated by forming the pattern on a photomask to a more complex shape than the actual circuit pattern. Such a complex pattern shape may be designed, for example, by incorporating optical proximity correction (OPC) into the actual circuit pattern. Also, attempts are made to apply the resolution enhancement technology (RET) such as modified illumination, immersion lithography or double exposure (or double patterning) lithography, to meet the demand for miniaturization and higher accuracy of patterns.
The phase shift method is used as one of the RET. The phase shift method is by forming a pattern of film capable of phase reversal of approximately 180 degrees on a photomask, such that contrast may be improved by utilizing optical interference. One of the photomasks adapted for the phase shift method is a halftone phase shift photomask. Typically, the halftone phase shift photomask includes a substrate of quartz or similar material which is transparent to exposure light, and a photomask pattern of halftone phase shift film formed on the substrate, capable of providing a phase shift of approximately 180 degrees between exposure light transmitted by a transparent section where no phase shift film is formed and exposure light transmitted by a (phase shift) section where the phase shift film is formed and having an insufficient transmittance to contribute to pattern formation. As the halftone phase shift photomask, there were proposed photomasks having a halftone phase shift film of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON) as disclosed in Patent Document 1, and photomasks having a halftone phase shift film of SiN or SiON.
There is a demand for further miniaturization of the mask pattern necessary for pattern transfer. For the purpose of enhancing resolution, a halftone phase shift photomask blank having a hard mask film deposited thereon is used because the hard mask film makes it possible to reduce the thickness of a resist film which is used for patterning in the processing of the photomask blank into a photomask. The hard mask film which is applied to the halftone phase shift photomask blank generally has a three-layer structure including a halftone phase shift film of a molybdenum silicon compound or silicon compound, a light shielding film containing chromium, and a hard mask film containing silicon deposited in order from the transparent substrate side.